7 General Relativity
139
and should be operational in 2020, and a fifth is under construction in India,
and expected to join the network in 2025. These detectors will improve the
accuracy of source location.
On September 14, 2015, half a century of perseverance since the time of
Weber finally paid off, when a gravitational wave was detected at the Hanford
and Livingstone observatories [8]. The Virgo observatory was offline at this
time undergoing upgrades. The signal arrived at Livingstone 0.007 s before
Hanford, indicating the source lay in the southern hemisphere.
Great care was taken to eliminate any possibility of error. The wave was
interpreted as arising from the final fraction of a second of the merger of two
black holes into a single, more massive, spinning hole, and provided the first
evidence that such phenomena exist. LIGO scientists estimated that the black
holes had masses of approximately 29 and 36 times that of the sun, and the
event occurred 1.3 billion years ago.
Since that historic date, detections have become almost commonplace. The
next twist in this exciting tale occurred on August 17, 2017 [9]. This time
it was not merging black holes that were detected, but two neutron stars
spiralling in and colliding with each other, only some 130 million light years
away. The masses of the neutron stars were 1.1 to 1.6 solar masses. (A similar
event was detected on 25th April, 2019.)
What was particularly exciting about this event was that optical, X-ray,
Gamma ray and radio telescopes were quickly pointed in the direction of the
source, as indicated by the LIGO/VIRGO measurements, and within 12 h
had located it, a fireball at the edge of a galaxy 130 million light years away.
Figure 7.12 shows a photograph of the galaxy taken by the Hubble Telescope.
The source is clearly visible. Over the 6 days following the photograph, the
source faded quickly away.
Bringing so many different types of observation platforms to bear on the
same source enables details of competing gravitational theories to be checked
against each other. The fact that electromagnetic signals (gamma-rays) and
gravitational waves arrive at the same time, (i.e. GW travel with the velocity
of light) is in agreement with Einstein’s theory and excludes some of the
alternative contenders for a theory of gravity.
With the development of further, more powerful, LIGO observatories,
gravitational waves have now emerged as a possible means of exploring
the physics of the early years of the universe, at a time period less than
380,000 years after the Big Bang, when electromagnetic radiation cannot
penetrate (see Chap. 11). This new field of astronomy has been named multimessenger astronomy, as it utilises widely different technologies. The future will
be watched closely to see what it brings.
139
and should be operational in 2020, and a fifth is under construction in India,
and expected to join the network in 2025. These detectors will improve the
accuracy of source location.
On September 14, 2015, half a century of perseverance since the time of
Weber finally paid off, when a gravitational wave was detected at the Hanford
and Livingstone observatories [8]. The Virgo observatory was offline at this
time undergoing upgrades. The signal arrived at Livingstone 0.007 s before
Hanford, indicating the source lay in the southern hemisphere.
Great care was taken to eliminate any possibility of error. The wave was
interpreted as arising from the final fraction of a second of the merger of two
black holes into a single, more massive, spinning hole, and provided the first
evidence that such phenomena exist. LIGO scientists estimated that the black
holes had masses of approximately 29 and 36 times that of the sun, and the
event occurred 1.3 billion years ago.
Since that historic date, detections have become almost commonplace. The
next twist in this exciting tale occurred on August 17, 2017 [9]. This time
it was not merging black holes that were detected, but two neutron stars
spiralling in and colliding with each other, only some 130 million light years
away. The masses of the neutron stars were 1.1 to 1.6 solar masses. (A similar
event was detected on 25th April, 2019.)
What was particularly exciting about this event was that optical, X-ray,
Gamma ray and radio telescopes were quickly pointed in the direction of the
source, as indicated by the LIGO/VIRGO measurements, and within 12 h
had located it, a fireball at the edge of a galaxy 130 million light years away.
Figure 7.12 shows a photograph of the galaxy taken by the Hubble Telescope.
The source is clearly visible. Over the 6 days following the photograph, the
source faded quickly away.
Bringing so many different types of observation platforms to bear on the
same source enables details of competing gravitational theories to be checked
against each other. The fact that electromagnetic signals (gamma-rays) and
gravitational waves arrive at the same time, (i.e. GW travel with the velocity
of light) is in agreement with Einstein’s theory and excludes some of the
alternative contenders for a theory of gravity.
With the development of further, more powerful, LIGO observatories,
gravitational waves have now emerged as a possible means of exploring
the physics of the early years of the universe, at a time period less than
380,000 years after the Big Bang, when electromagnetic radiation cannot
penetrate (see Chap. 11). This new field of astronomy has been named multimessenger astronomy, as it utilises widely different technologies. The future will
be watched closely to see what it brings.